This invention concerns a process for preparing phenoxybenzoic acid derivatives which are useful as herbicides, and in particular, acifluorfen, i.e., the compound 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid, which in the form of its sodium salt is commercially available as the product Tackle (Mobil Oil Corporation). Acifluorfen, which possesses the structure ##STR1## and related phenoxybenzoic acid derivatives constitute a class of highly effective herbicides for the post-emergence control of a variety of weeds such as certain broadleafs associated with soybeans.
Copending U.S. patent application Ser. No. 208,081 filed Nov. 18, 1980 and its predecessor applications (Ser. Nos. 067,508, filed Aug. 17, 1979 and 051,254, filed June 22, 1979, both now abandoned) each describes a four-step synthesis of a class of phenoxybenzoic acid derivatives to which acifluorfen belongs. The sequence of steps comprising this synthesis is as follows: ##STR2## X.sub.1, X.sub.2 and X.sub.3 each is H, F, Cl, Br, CF.sub.3, OCF.sub.2 CHZ.sub.2 (Z=Cl, F or Br), OCF.sub.3, CN, CO.sub.2 R (R=lower alkyl), --C.sub.6 H.sub.5, O-alkyl, NO.sub.2 or SO.sub.2 (lower alkyl and X.sub.4 is F, Cl or Br, provided that at least one of X.sub.1, X.sub.2 and X.sub.3 is other than H. ##STR3##
The carboxyl group in the product compounds can be made to undergo transformation to a variety of other groups, including salts, employing conventional procedures.
In oxidation step 3, the catalyst system comprises a source of cobalt compound and a source of bromide, compound, e.g., a combination of cobaltous acetate tetrahydrate and sodium bromide. The oxidation is carried out in a suitable inert solvent, e.g., any of the lower aliphatic carboxylic acids such as acetic acid, and in the presence of a free radical initiator, e.g., any of the peroxides such as hydrogen peroxide. Favorable process economics require an efficient system for catalyst and solvent recovery in this step. In accordance with the recycle procedures of the aforesaid U.S. patent applications, the cobalt compound and part of the bromide compound can be recovered and recycled by crystallizing the phenoxybenzoic acid derivative from the reaction solution, collecting the crystallized product on a basket centrifuge and recycling the mother liquors which contain the catalyst solvent and a few weight percent of phenoxybenzoic acid derivative to the oxidation step. The water of reaction can be removed from the recycled mother liquors by conventional means, e.g., fractional distillation or addition of acetic anhydride. The water is advantageously equal to or less than 1 weight percent in the recycled mother liquors and equal to or less than 2 weight percent after the addition of aqueous peroxide. The catalyst and acetic acid remaining on the phenoxybenzoic acid filter-cake can be recovered by dissolving the filter-cake in a suitable solvent such as methylene chloride and extracting this solution with water. The acetic acid, cobalt acetate, and sodium bromide partition into the aqueous phase which can then be added to the recycled mother liquors. Approximately 88 weight percent of the acetic acid and more than 95 weight percent of the cobalt and bromide are recovered in each recycle employing the foregoing procedures.